Abstract
In order to understand mechanisms of strategy switching in the stabilization of unstable dynamics, this work investigates how human subjects learn to become skilled users of an underactuated bimanual tool in an unstable environment. The tool, which consists of a mass and two hand-held non-linear springs, is affected by a saddle-like force-field. The non-linearity of the springs allows the users to determine size and orientation of the tool stiffness ellipse, by using different patterns of bimanual coordination: minimal stiffness occurs when the two spring terminals are aligned and stiffness size grows by stretching them apart. Tool parameters were set such that minimal stiffness is insufficient to provide stable equilibrium whereas asymptotic stability can be achieved with sufficient stretching, although at the expense of greater effort. As a consequence, tool users have two possible strategies for stabilizing the mass in different regions of the workspace: 1) high stiffness feedforward strategy, aiming at asymptotic stability and 2) low stiffness positional feedback strategy aiming at bounded stability. The tool was simulated by a bimanual haptic robot with direct torque control of the motors. In a previous study we analyzed the behavior of naïve users and we found that they spontaneously clustered into two groups of approximately equal size. In this study we trained subjects to become expert users of both strategies in a discrete reaching task. Then we tested generalization capabilities and mechanism of strategy-switching by means of stabilization tasks which consist of tracking moving targets in the workspace. The uniqueness of the experimental setup is that it addresses the general problem of strategy-switching in an unstable environment, suggesting that complex behaviors cannot be explained in terms of a global optimization criterion but rather require the ability to switch between different sub-optimal mechanisms.
Highlights
Using a tool or shaping a new tool for solving a task are important aspects of sub-symbolic human cognition, but this is not a prerogative of humans: primates [1,2,3] and even crows [4] can exhibit such skilled behavior
Panels A, A1, A2 refer to the stiffness stabilization strategy (SSS) and panels B, B1, B2 to the positional stabilization strategy (PSS); dotted curves are related to the early phase of training and continuous curves to the late phase
This is a preliminary but relevant piece of information to understand how the brain can switch among different strategies during complex motor tasks, such as when car drivers switch from one gear to another in different driving conditions
Summary
Using a tool or shaping a new tool for solving a task are important aspects of sub-symbolic human cognition, but this is not a prerogative of humans: primates [1,2,3] and even crows [4] can exhibit such skilled behavior. If a task is stable and the tool is sufficiently rigid, the incorporation of the tool in the body schema and its control require a rather straightforward reorganization of the coordination patterns, equivalent to the modification of the Jacobian matrix of the end-effector. Unstable tasks are common components of human activities, like screwing/unscrewing, drilling, inserting a peg in a hole, chiselling, manipulating soft tissues (like in surgery), balancing a pole etc. This study addresses both issues (underactuated control and unstable task) at the same time, further developing a previous work [7] in which naıve subjects were exposed to the unknown tool in an unknown environment and managed to find out suitable stabilization strategies
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